Electric power is typically conveyed from electric power generators to users via a network of transmission and distribution circuits. Electric power is commonly generated as three-phase alternating current (AC) at a frequency of 50 Hz or 60 Hz. Each phase requires a current-carrying wire, and has voltage and current nominally lagging or leading any other phase by 120 degrees. Power is generated, for example, at 4 kV voltage, stepped up to 128 kV or 750 kV for transmission over long distances, and then stepped down in stages to 4 kV or 33 kV for distribution to various neighborhoods. Voltage may further be reduced, by pole-mounted or pad mounted transformers, for delivery at 120V and 240V to residential and commercial users within these neighborhoods.
Large voltage transformations typically take place at transmission or distribution substations. Functions of the substations may include voltage transformation, regulation and control, power-factor (e.g. capacitor-bank) and load balancing, monitoring, and protection of hardware. Proper monitoring and protection is extremely important in preventing damage to equipment, reducing hazards and minimizing the number of users who may have to be disconnected from an electric distribution network due to damage or equipment failures. Conventional protection systems include fuses and relays, each having predetermined response times and zones of control to minimize propagation of failures.
Fuses are located throughout the electric distribution network and disconnect circuits experiencing excessive current flow due to equipment failure, storm damage, etc. Each fuse is selected with a predetermined response time to accommodate the need of the circuit being protected. The fuse does not blow under normal operation or a momentary over-current condition, but is designed to blow under a true, sustained fault situation. Relays are similarly used to detect faults and initiate disconnection of faulted circuits; relays, however, are typically more complex than a fuse. A relay typically includes a voltage and/or current sensor and a set of electrical contacts driven by the sensor. Electrical contacts in the relay may be connected to circuit breakers which, in turn, physically disconnect faulted lines or circuits when the breaker is opened (xe2x80x9ctrippedxe2x80x9d). Modern relays may use solid state switches in place of electrical contacts.
There are many types of relays, each having different characteristics. Depending on its electrical and physical characteristics, the response time of the relay may be varied. For example, a relay may be made to trip a breaker in one or two cycles of the 50 Hz-60 Hz frequency when it detects excessive current. This type of relay is known as an instantaneous over-current (OC) relay (Type 50). Another type of relay is the time-over-current (TOC) relay (Type 51). The TOC relay may include an adjustable delay so that it may respond quickly to large over-current conditions, but more slowly to small over-current conditions. Predetermined response curves (TOC Curves) are usually provided by the manufacturer of the relay to aid in the selection and adjustment of a Type 51 relay.
Small current overloads in a local circuit may be tolerated if the magnitude and duration of the overload are not expected to damage the distribution network. For example, temporary overloads lasting a few hours may be acceptable in order to maintain service to users during a hot summer day when peak load periods are expected. Concurrent with maintaining service during expected overload conditions, the relay must still effectively protect the network in the event of a true, sustained circuit fault. Some networks may use a Type 50 relay in parallel with a Type 51 relay to provide better response to large fault currents, while not over-reacting to small, temporary overloads.
Relays are installed at various locations in a network. With respect to electromechanical relays, each protective function for one phase of a circuit generally requires a separate relay. Providing separate relays for each function per phase is expensive, because of space requirements and installation/wiring costs for so many relays. More recent designs provide for microprocessor-based relays. Microprocessor-based relays are able to combine protective functions for all three phases into one unit. Furthermore, microprocessor-based relays may be remotely reset and adjusted to provide responses that vary depending on the nature of the electrical load during the year. For example, TOC curves and operating points may be changed in anticipation of changes in loads.
A conventional relay, either electromechanical or microprocessor, only senses current or voltage on a circuit to which the relay is connected and only disconnects a breaker for that circuit. The relay does not communicate its measurements with any other relay during the period in which it senses a fault and trips the breaker. Some relays may communicate status information, but do not share measurements to make protective decisions. Using TOC curves, for example, is one of the important ways to limit the relay""s xe2x80x9czone of controlxe2x80x9d. For example, relays protecting the spokes or xe2x80x9cfeedersxe2x80x9d of a radially distributed power network may be adjusted to respond faster (and at lower trip currents) than relays protecting the hub of the network. In this manner, a faulted feeder may be disconnected before disconnecting the hub and, consequently, all the remaining feeders.
FIG. 1 illustrates a conventional electric utility network protected by relays. As shown, a 128 kV transmission line feeds the primary windings of each of two power transformers 30 and 31 by way of circuit breakers 18 and 24, respectively. The secondary of each of the two power transformers 30 and 31 feeds electric power at 13 kV to feeder bus 1 and feeder bus 2, respectively. Feeder bus 1 transmits electric power at 13 kV by way of two circuit breakers 44 and 52 to two feeders, feeder #1 and feeder #2. Similarly, feeder bus 2 transmits electric power by way of two circuit breakers 47 and 55 to two feeders, feeder #3 and feeder #4. Feeders #1-#4 provide power, for example, to a neighborhood, factory or shopping center. Tie-breaker 40 provides an alternative path of electric power in the event that power transformer 30 or power transformer 31 is taken out of service. It will be appreciated that each power line shown in FIG. 1 represents three power lines corresponding to the three phases of electric power. Depending on the network, there may actually be three times the number of breakers and relays shown in FIG. 1.
Also shown in FIG. 1 are current transformers 14, 16, 32, 34, 41, 42, 50 and 51 feeding current to various relays. The current transformers each provide an output current in proportion to the current flowing through each line. For example, the current flowing through a line may be 1200 amperes, whereas the corresponding current transformer may provide an output of 5 amperes. As shown, current transformer 14 provides current to OC relay 20, TOC relay 21 and differential relay 22. Differential relay 22, which also senses current from current transformer 32, reacts to an imbalance between current flowing into and out of power transformer 30. OC relay 20, TOC relay 21 and differential relay 22 control breaker 18, and each may individually trip the breaker if a predetermined condition occurs. Similarly, OC relay 26, TOC relay 27 and differential relay 28 control breaker 24. OC relay 45 and TOC relay 46 control breaker 44. OC relay 48 and TOC relay 49 control breaker 47. OC relay 53 and TOC relay 54 control breaker 50. OC relay 56 and TOC relay 57 control breaker 55.
Also shown in FIG. 1 are under-voltage relays 37 and 39 connected to potential transformers 36 and 38. Under-voltage relays 37 and 39 are shown connected to the secondary of each potential transformer to protect against transformer failures or other failures in the circuits. A potential transformer provides an output in proportion to voltage on the feeder bus, but at a stepped down voltage level. For example, a bus voltage of 13 kV may be stepped down to an output voltage of 120V. Two additional relays shown are under-frequency relays 60 and 62 controlling breakers 18 and 24, respectively. An under-frequency relay protects against excessive line frequency deviations.
As discussed above, conventional protection in a network requires that relays be selected and adjusted so that a fault in the network may be contained. For example, if there is a fault on feeder #1 at a local neighborhood (at the spoke level), relays 45 or 46 should trip breaker 44 before relay 20, 21, 22, 37 or 60 causes breaker 18 to trip (at the hub level). If the relays have been adjusted properly, feeder #1 is disconnected, but feeder #2 continues to provide electric power. If the relays on feeder #1, however, do not trip their associated breaker fast enough, the relays ahead of power transformer 30 will likely trip their associated breaker, thereby cutting power to both feeder #1 and feeder #2. Similarly, if the relay ahead of power transformer 30 do not trip due to a fault at the power transformer or on feeder #1, the entire transmission line and substation may be shut down by another relay positioned higher in the hierarchy of the network.
A need exists, therefore, for an apparatus and method for improved detection of and protection against electric faults in a power network. A need also exists for eliminating the complexity of selecting and adjusting the tripping characteristics for relays and breakers in a power network. A need also exists for an improved method for coordinating and sequencing protective actions and breakers in a power network.
To meet this and other needs, and in view of its purposes, the present invention provides a distributed monitoring and protection system for a distributed power network. The power network has a plurality of lines for transmitting electric power from a station with circuit breakers included in the lines. The distributed monitoring and protection system includes at least one monitoring unit coupled to at least one of the plurality of power lines for measuring electrical parameters of the power line; and at least one control unit communicating over a data network with the monitoring unit and receiving measured electrical parameters from the monitoring unit. The control unit includes a processor for analyzing the measured electrical parameters and tripping at least one of the circuit breakers when a fault in the power network is determined.
The measured electrical parameters include values as a function of time for current, voltage, power, frequency, power factor, and harmonic components of the current and voltage in the power line. The measurements are obtained by the monitoring unit, communicated over the data network, and analyzed by the control unit in a sufficiently short time to activate protective devices and thereby limit the effects of a fault in the power network. Preferably, the measurement and communication times are each one cycle or less of the alternating current power network.
It is understood that the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.